The rotational speed of a dc motor decreases when the mechanical load of the motor increases. At least, this is true when no countermeasures are taken. In the case of a shaver with a rotating motor, the decrease in motor speed translates in reduced shaving performance
In the GB 2 435 413 A, a drive circuit for a hair clipper is described. The hair clipper comprises an electric motor and a control circuit for operating the motor at a constant speed regardless of load condition. Constant clipper speed is achieved by increasing the voltage in response to a sensed increase in load current, caused by increased work required of the clippers due to eg. lack of lubrication. The control circuit comprises detector means for sensing the supply voltage to the clipper and the clipper current, processing means for generating an error signal indicative of a change in supply voltage needed to maintain a constant motor speed and a pulse width modulation (PWM) module responsive to the error signal for adjusting the supply voltage to the clipper to drive the motor at said constant speed.
Such load compensation electronics have been developed to compensate the speed change of a shaver dc motor under load. However, it was observed that the motor did not run constantly and smoothly any more. This could be measured under constant load. The effect was in particular present when the dc motor was running without load, i.e. without really shaving in that moment.